In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, handover is a vital part of any cellular communications network. A handover may be defined as a process of transferring an ongoing connection of a wireless communication device from one radio access network node, denoted as a serving radio access network node, to another radio access network node, denoted as a target radio access network node, in order to accomplish a transparent service over a large coverage area. The handover should be performed without any losses of data transmission to/from the wireless communication device and with as small interrupt as possible for the wireless communication device.
To enable a handover, it is necessary to find a suitable target cell as served by the target radio access network node, and to ensure that it is possible to sustain reliable communication to/from the wireless communication device in the target cell. Candidates for suitable target radio access network nodes and/or target cells are usually stored in so-called neighbor lists, which are stored at least at the serving radio access network node. To make sure that it is possible to sustain reliable communication to/from the wireless communication device in the target cell, the connection quality in the target cell needs to be estimated before the handover can be performed.
The connection quality of the target cell is commonly estimated by measurements related to the wireless communication device. Downlink (DL), i.e. transmission from the radio access network node to the wireless communication device, and/or uplink (UL), i.e. transmission to the radio access network node from the wireless communication device, measurements may be considered. Relying solely on uplink measurements may be unreliable, since the uplink connection quality can be different from the corresponding downlink connection quality. Therefore, handovers in cellular communications networks are commonly based on downlink measurements.
In existing cellular communications networks, all radio access network nodes (Network nodes) continuously transmit pilot signals that wireless communication devices in neighbor cells use to estimate the target cell quality. This is true in the Global System for Mobile Communications (GSM) where such pilot signals are transmitted on the broadcast control channel (BCCH), in the Universal Mobile Telecommunications System (UMTS) where such pilot signals are transmitted on the Common Pilot Channel (CPICH) and in the Long Term Evolution (LTE) telecommunications system where such pilot signals are transmitted as cell specific reference signals (CRS), as well as in WiFi where such pilot signals are transmitted as beacons. This makes it possible to estimate the quality of neighbor cells with relatively good accuracy. The wireless communication devices perform measurements periodically and report the measurements to the network. If it is detected that the serving cell quality is getting near the candidate target cell quality, a more detailed measurement process or a handover procedure may be initiated.
Future cellular communications networks, e.g. 5th generation (5G) system, may use advanced antenna systems to a large extent. With such antenna systems, signals will be transmitted in narrow transmission beams to increase signal strength in some directions, and/or to reduce interference in other directions. When the antenna system is used to increase coverage and signal quality, handovers between narrow transmission beams, either to another beam within the current serving network node, or to a beam from other candidate target network node, may become a necessity. The serving network node also needs to decide if a beam switch or beam update is necessary within its own cell. The transmission beam through which the network node is currently communicating with the wireless communication device is called the serving beam and the transmission beam it will hand over to, or switch to, is called the target beam. The serving beam and the target beam may be transmission beams of the same or different network node.
Applying the principle of continuous transmission of pilot signals in the existing cellular communications networks to the transmission of mobility reference signals (MRS) in all individual transmission beams in such a future cellular communications network may be convenient for mobility measurements performed by the wireless communication devices, but it may degrade the performance of the network. For example, continuous transmission of MRS in all individual transmission beams may generate a lot of interference in the network, it may consume network capacity which is otherwise available for data, and may lead to higher power consumption of the network, since there are large numbers of narrow transmission beams.
Further, in a communications network relying on advanced antennas with narrow transmission beams to improve coverage, it is inefficient, or sometimes even impossible, to transmit the MRS in all transmission beams at the same time. The natural alternative to transmit consecutively in different beams leads to longer measurement periods, slower handovers and beam updates.
In addition, the network makes handover decisions based on mobility measurement reports sent by the wireless communication devices. The quality and accuracy of the measurements depend largely on the MRS allocation. For example, a sufficient number of MRS resource elements (REs) must be available per measurement to achieve a sufficient measurement signal to noise ratio at the wireless communication device. As another example, a MRS placement with a short span of the REs, the measurements may be unduly affected by momentary fading.
Hence, there is a need for an improved network node and method therein for allocation and transmission measurement reference signals in the future communications networks for mobility measurements performed by wireless communication devices.